Photoluminescence imaging and spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials, such as silicon semiconductor wafers, solar cells, as well as other workpieces and materials. In a typical photoluminescence process, light is directed onto a wafer or other workpiece (hereinafter collectively referred to as a “sample”), where at least some of the light is absorbed. The absorbed light imparts excess energy into the material via a process of “photo-excitation.” This excess energy is dissipated by the sample through a series of pathways; one such pathway is the emission of light, or photoluminescence. The intensity and spectral content of this photoluminescence is directly related to various material properties of the sample.
Photoluminescence imaging processes may be used to identify and quantify defects and contaminants present in the sample based on spatial variations in the photoluminescence images produced. One photoluminescence imaging process, as described in International Application Number PCT/GB97/02388 (publication number WO 98/11425), which is incorporated herein by reference, involves probing the surface and/or the sub-surface bulk region of the sample with one or more lasers of varying excitation wavelengths. A laser of a given wavelength is directed into the sample and penetrates the sample to a given depth. Return light emitted from excited regions of the sample is detected and quantified by a detection system. Images of the measured return light, including spatial images of defects and contaminants in the sample, may then be produced by the detection system or by an associated image-producing system.
Samples, such as solar materials or cells when tested using photoluminescence imaging, may reflect a significant portion of light used for sample excitation, e.g., approximately 1%-35% of the excitation light is reflected. At the same time, photoluminescence radiation generated in the sample may be significantly lower in intensity than the excitation light, e.g., by multiple orders of magnitude (>10). As a result, photoluminescence radiation is heavily energy “contaminated” by the reflected excitation light. Conventional filters do not have enough attenuation to adequately eliminate the excitation light. Thus, there is a need to filter the primary excitation light from the photoluminescence signal.